As is known, data conversion circuitry converts analog signals into a discrete digital format which can then be processed by a digital signal processor. The analog-to-digital conversion may be accomplished by a variety of circuits. For example, one such circuit, which is called sigma-delta converter, is shown in FIG. 1.
FIG. 1 illustrates a single-stage, single-bit, sigma-delta converter. In the first stage of the converter, a voltage reference signal (V.sub.out) is selectively combined with an incoming analog signal to produce a combined signal. Typically, the voltage reference is either added or subtracted from the incoming analog signal depending upon the level of the analog signal and the prior state of the digital output of the converter. The combined signal then serves as an input to an integrator which integrates the combined signal. The integrator output then serves as an input to an analog-to-digital (A/D) element which produces a digital output signal at each clock cycle. Typically, when the input of the A/D element exceeds a threshold, the A/D element produces a logic high output signal. Alternatively, when the input does not exceed the threshold, the A/D element produces a logic low output signal. Based upon the output of the A/D element, a feedback path selectively alters the manner in which the voltage reference is combined with the analog signal in an attempt to force the voltage across the integrator's inputs to zero. In the circuit shown in FIG. 1, the digital output signal is used to control the circuitry which selectively combines the voltage reference signal with the analog input signal. Hence, the digital output signal determines the polarity of the reference signal that is combined with the analog input signal.
As is known in the art, a variety of circuits and techniques may be used to convert an analog input signal to an equivalent digital output signal, each with its relative benefits and drawbacks. While the converter illustrated in FIG. 1 is a single bit converter, the conversion technique may be expanded to multiple-bit conversion processes, successive approximation processes, and other processes. A common requirement among all of the processes, however, is the requirement of a voltage reference of a fixed voltage level that is used to selectively combine with the analog input signal. In order for the data conversion circuitry to perform optimally, the voltage reference must be noise free and temperature invariant.
A circuit commonly used to produce a temperature invariant voltage reference is also illustrated in FIG. 1 and occupies the upper left hand portion of FIG. 1 and is designated as a voltage reference. The circuit includes two current-driven bipolar transistors, a switched capacitor network, and a summing amplifier. The bipolar transistors, driven at two different current densities, produce differing base-to-emitter voltages (V.sub.BE). Because the base-to-emitter voltage exhibits a negative temperature coefficient and the difference between the base-to-emitter voltages of the transistors .DELTA.V.sub.BE exhibits a positive temperature coefficient, the signals may be selectively combined to produce a temperature invariant voltage reference. As illustrated in FIG. 1, a switched capacitor array and a summing amplifier may be employed to produce the temperature invariant voltage reference (referred to as V.sub.out) according to the equation shown.
A shortcoming of the standard temperature invariant voltage reference circuitry relates to the circuit components used. The voltage reference circuitry requires both a switched capacitor network and a summing amplifier to create the temperature invariant voltage reference. These components not only require additional components on the chip but add complexity, consume power, and generate additional heat.
Further, in a typical integrated circuit that includes an A/D converter, the voltage reference circuitry resides at a location removed from the A/D converter. Thus, noise couples onto the voltage reference transmission path and resultantly becomes a part of the voltage reference. Noise in the voltage reference negatively affects the performance of the conversion process. Therefore, a large external bypass capacitor is usually employed to reduce the noise on the voltage reference. The external bypass capacitor also improves the power supply rejection ratio of the converter. Unfortunately, two pin-outs must be added to the chip and additional footprint area is needed to accommodate the bypass capacitor.
Still further, by examining the circuit of FIG. 1, it can be seen that for a single stage sigma-delta converter, two operational amplifiers are needed: One for the voltage reference and a second for the data converter section. As mentioned above, by utilizing the circuit of FIG. 1 additional components are needed which has the above mentioned drawbacks.
Therefore, a need exists for a converter that reduces component count, reduces complexity, reduces power consumption, and eliminates the need for the external pins to filter noise from the voltage reference while still providing a reliable analog-to-digital conversion.